Clean up code, following r1621978.

* subversion/libsvn_wc/diff_editor.c
  (filter_file_opened): Simplify by using an existing function. Remove a
    debugging print.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1621981 13f79535-47bb-0310-9956-ffa450edef68